The present invention relates generally to traffic control systems, and more particularly to improvements in operating traffic signal lights at controlled roadway intersections during times when loss of electrical power occurs.
Operation of a traffic control system requires a supply of AC power to equipment. When AC power ceases to be supplied, the traffic control system ceases to operate and the traffic signal lights no longer emit light, thereby becoming dark. The result is that drivers of vehicles approaching the signalized intersection do not see any traffic signal lights. The drivers of vehicles approach what is typically referred to in the industry as “a dark intersection.” The Manual on Uniform Traffic Control Devices (MUTCD) of the Federal Highway Administration (FHWA), listed in the Federal Register, states that it is acceptable to operate a signalized intersection as “a dark intersection” and that, during such operation, drivers of vehicles are expected to interpret “a dark intersection” the same as they would an intersection having stop signs; stopping their vehicles before proceeding through the intersection. In practice, since it is commonly ignored, some agencies require that police be dispatched to intersections to ensure 4-way stop. As should be appreciated, this is very labor intensive/expensive. Still other agencies require DPW (department of public works) personnel to take temporary stop signs out to intersections when power is out. This is very labor intensive/expensive as well. Not only must personnel place temporary stop signs at intersection, but personnel must remove the temporary stop signs as soon as power comes back on.
One of the purposes of traffic signals is to make intersections more visible, and hence, safer. There has long been need for improvements in making “a dark intersection” more visible to drivers of vehicles for traffic control. Traffic signal lights may be difficult to see even when operating properly and lit. But when traffic signal lights are dark, intersections become very difficult for drivers of vehicles to see. The result is that accidents occur, causing property damage and bodily harm with potential loss of life. Power outages leading to loss of traffic signal light operation are most likely to occur as the result of inclement weather, which causes visibility to be degraded. Thus, the loss of traffic signal operation most commonly occurs when its reliable operation is needed most. Loss of traffic signal light operation during nighttime due to loss of power poses an all-too-common threat to the safety of drivers of vehicles, their passengers and bystanders.
While controller failures have been proposed which initiate an alarm upon detection of a problem, this is often too late and can also be very dangerous. For example, if two or more cars are approaching an intersection from different directions, and the traffic lights suddenly go out, the approaching cars would not know who had the right-of-way and could crash.
Various systems have been proposed for providing computers with back-up power, such as can be provided by a bank of batteries coupled in series and an inverter, such that the traffic signals can continue to operate as normal under a back-up power. Such systems often include battery chargers for charging the bank of batteries to a predetermined float voltage, the float voltage being determined by the sum of the voltages of the batteries. However, if one, or more, of the batteries, or cells thereof, are defective and, hence, effectively a short, the total voltage across the bank of batteries will never reach the float voltage to shutoff the battery charger, resulting in a damaging overcharging of the remaining good batteries.
Further, when the utility power returns, it is necessary for the controller to transparently switch from the backup power to the utility power, i.e., the output of the inverter must be in-phase with the utility power.
Still further, systems often shut down when the line voltage is determined to be too low. However, utility lines often have relatively high impedance, and line conditioners to condition utility power often are highly inductive. The high inductance results in a large inrush of current upon a restart of the controller, and this large inrush current traveling through the high impedance utility line results in a short term voltage drop which can trick the system into shutting down.
Finally, inverters typically include power transistors. To monitor the condition of a load coupled to the inverter, devices have been proposed which monitor the current through the power transistors when conducting. This, however, does not always provide an accurate indication of the condition of the load.
As an alternative to complete back-up power or alarms during power failure, traffic control systems have used a controller unit that energizes load switches that drive the signal lamps through a flash transfer relay. In the event that a power failure should arise, a monitor actuates the relay to transfer the traffic signal loads to a flasher module. When this transfer occurs, the controller unit and load switches are disconnected from causing the traffic signal lights to be turned ON and to be turned OFF.
The flasher module is capable of causing the traffic signal lights to alternate regularly OFF and ON. This is accomplished by the flasher module in such manner that traffic signal lights are flashed ON and OFF. In doing so, drivers of vehicles may see flashing red traffic signal light indications at the intersection, indicating for them to stop before proceeding through the intersection in a safe manner.
A traffic control system is normally considered as consisting of a traffic controller unit for the purpose of providing 24 volt DC input signals to one or more load switches used to turn traffic signal lights ON. A conflict monitor device is used to monitor the presence of proper alternating current field wire voltages supplied to power the traffic signal lights. When improper AC voltages exist, the conflict monitor causes an electromechanical relay to operate, which in turn causes the flash transfer relay to remove traffic signal light power from the load switches and to connect the traffic signal light power to a flasher unit, which causes traffic signal lights to flash ON and OFF.
Back-up power supplies with power storage capability have been used in traffic control systems at signalized intersections to maintain operation of the traffic control system as it would operate from the external AC power source. Use of such back-up power sources has been limited to only a few signalized intersections, due to space limitations and their high cost.